I feel you. When I was a kid I was extremely active, cheerful, friends with everyone at school, stood up to bullies bothering my friends and always sought company. Then shit happened and gave me some mf huge social anxiety that basically turned me into an extreme introvert. At first I wouldn't even talk to strangers. My teenagehood has been utter shit because of this.

As I'm getting older and getting my self-confidence back, I'm more and more assertive and getting back to who I used to be, but I'm still not there yet. I'm still very introverted in my behavior and drained by too much social interaction, something that definitely wasn't the case in my childhood. I believe I was supposed to be an extrovert but then shit happened. Anyways I'm working hard to claim my confidence back. It's a very long process, it's been taking years and still will be, but it's something I had as a kid so I don't see why I can't have it anymore now.

The theory says that your type can't change, and I can understand why, but then I can't really explain what happened to me here other than a type change.

Can't really give you the solution to your problem, as I have the same. But you're definitely not alone in this. I'm wishing you the best :)